Keeping pace with the progress of materials for advanced technologies in recent years, there is a growing demand for base resins of increasingly higher performance for such materials. In the area of encapsulation of semiconductors, for example, the packages are becoming smaller in thickness and larger in area and the technique of surface mounting is coming into wider use in response to the trend of higher-density packaging in recent years. As the result, cracking of packages has caused a serious problem and there is a strong demand for availability of base resins with improved properties in respect to moisture resistance, heat resistance and adhesiveness to metallic substrates. From the standpoint of reduction of environmental load, the trend of public opinion is to exclude the use of halogen-containing flame retardants and there is a demand for base resins of improved flame retardance.
However, none of existing epoxy resins is known to satisfy the aforementioned property requirements. For example, universally known bisphenol type epoxy resins are widely used as they are liquid at room temperature, easy to work with and readily mixable with curing agents and additives, but they present problems concerning heat resistance and moisture resistance. Novolak type epoxy resins are known for improved heat resistance, but they still have problems concerning moisture resistance and adhesiveness. Furthermore, the conventional epoxy resins whose backbone is composed of hydrocarbons alone completely lack flame retardance.
As a means to improve flame retardance without the use of halogen-containing flame retardants, addition of flame retardants based on phosphate esters is disclosed in JP09-235449-A, JP10-182792-A and elsewhere. However, the use of flame retardants based on phosphate esters does not provide sufficient moisture resistance. Moreover, phosphate esters undergo hydrolysis in an environment of high temperature and high moisture thereby adversely influencing the reliability as insulating material.